A conventional protective device, mounted on say a secondary cell device, has a protective function not only against over-current but also against over-voltage. This protective device includes a heating member and a fusible conductor layered on the heating member via an insulation layer. The fusible conductor is formed by a segment of a low melting metal and may be fused off by over-current. In case of an over-voltage, current is supplied to the heating member in the protective device, and the fusible conductor is fused off due to heating of the heating member. The fusible conductor may be fused off as a result of high wettability of the fusible conductor of a low melting metal in the fused state against the surface of the conductor layer the fusible conductor is connected to. The low melting metal in the fused state is drawn close to a conductor layer, such as an electrode, as a result of which the fusible conductor is fused off to break the current.
On the other hand, in keeping up with reduction in size of the electronic equipment, such as mobile equipment, reduction in size or thickness and stability of the operation as well as a high operating speed may be beneficial. In light of this, a protective device having a fusible conductor of low melting metal is arranged on an insulation substrate and sealed with an insulation cover, and in which the fusible conductor is coated with a flux. This flux is provided to prevent oxidation of the surface of the fusible conductor and to allow the fusible conductor to be fused off promptly and stably at the time of heating of the fusible conductor.
Such a type of the protective device is shown in FIGS. 13 and 14. This protective device includes a heating member 2 of a resistance material between a pair of electrodes 5a provided on both ends of a base substrate 1. A conductor layer 4 connected to one of the electrodes 5a is provided on top of the heating member 2 via insulation layer 3. Another pair of electrodes 5b is provided on the lateral sides of the base substrate 1. A fusible conductor 6, formed by a low melting metal piece, is connected between the electrodes 5b by a solder paste 7. The fusible conductor 6 is also connected to an underlying conductor layer 4 by the solder paste 7. A flux 8 is coated on the fusible conductor 6 on the base substrate 1, and an insulation cover 9 is mounted to overlie the base substrate 1.
The fusion/disruption of the fusible conductor 6 of the low melting metal due to over-current may occur as follows: When the fusible conductor 6 is fused, the fusible conductor 6 in the fused state is drawn close to the conductor layer 4 and the electrodes 5b, because of wettability of the fusible conductor 6 with respect to the surfaces of the electrodes 5b or the conductor layer 4 the fusible conductor is connected to. As a result, the fusible conductor 6 between the electrodes 5b is disrupted to break the current. Hence, this wettability markedly influences the current breaking characteristic.
A protective device, improved in fusion characteristic in light of the wettability and the aggregation performance at the time of fusion/disruption of the fusible conductor, is disclosed in Patent Document 1. The protective element includes an insulation substrate, a pair of electrodes mounted spaced apart from each other on the surface of the insulation substrate, and a fusible alloy conductor connected between the pair electrodes. The protective element also includes a flux deposited on the fusible alloy conductor and an insulation/sealing material that overlies the flux. An underlying layer, whose wettability against the fusible alloy conductor in the fused state is smaller than that of the insulation substrate, is formed at the fusible alloy conductor forming position. When the fusible alloy conductor is fused, the fused alloy conductor is flipped by the underlying layer and hence is disrupted promptly. Moreover, no sparking is produced at the time of fusion/disruption. The fusible alloy may readily be aggregated by its surface tension onto the electrode to ensure reliable disruption.
Another protective device to shorten the circuit breaking time due to aggregation of the low melting metal at the time of fusion/disruption is disclosed in Patent Document 2. In Patent Document 2, two or more strands of low melting metal are provided between a pair of electrodes designed to cause the current to flow through the low melting metal. In so doing, the low melting metal between the electrodes is separated into independent sections to increase the number of fusion/disruption start points in the low melting metal to have the operating time shortened and improve stability.